The present invention relates in general to solar energy water heaters and has specific reference to automatic means for protecting such heaters against the ill effects of frost.
In regions or countries where outdoor temperatures may fall below the water freezing point, some protection means must be provided to prevent the water from freezing in solar energy collectors and in the pipings leading to and from such systems. Since thermal insulation is not perfect, the expansion of the water due to the freezing thereof may cause serious damage to occur in the complete plant.
When selecting among known means for solving this problem, the following three solutions are generally proposed:
adding an anti-freeze substance to the water contained in the solar energy heater, by keeping the anti-freeze and water mixture within a closed loop comprising a heat exchanger adapted to transfer the heat received from the sun radiation to the water to be used in the system. As a result, the efficiency of the system is reduced, its cost is increased, and the anti-freeze and water mixture is likely to leak, due to a possible lack of fluid-tightness in the heat exchanger. For this last-mentioned reason, in many countries, the use of anti-freeze and water mixture in sanitary water heating systems is strictly forbidden; PA1 adding heat to the solar energy elements by creating a circulation of luke warm water therein, or energizing electric Joule-effect heating resistances. This additional heat produced before the internal temperature of the solar energy heater has reached the water freezing point constitutes an energy-consuming solution; PA1 draining automatically the water contained in the solar energy water heater when the temperature within the solar energy system approaches the water freezing point. The degree of precision of the temperature measuring instrument, located preferably inside the solar energy collector, which delivers the signal for draining water out of the system when the temperature falls below a predetermined threshold, is not reliable, and, like all measuring instruments, displays a magnitude which may depart from the measured magnitude, within limits depending, of course, on its precision.
As a result, this last-mentioned solution is attended by a twofold inconvenience. On the one hand, there is a risk of bursting the water heating elements, as a consequence of water expansion due to the frost effect, in case the predetermined temperature threshold from which the system must be drained is nearer to the water freezing point than the absolute value of the precision of the temperature measuring instrument associated with the system. On the other hand, to avoid this risk of bursting the system as a consequence of water freezing, the care consisting in setting the predetermined temperature threshold from which the system has to be drained at a value relatively remote from the water freezing point is objectionable, in that it increases unduly the corrosion effect in the solar energy heater, due to the alternation of water and air in the fluid circuit.